Method and system for transferring calibration data between calibrated measurement instruments

ABSTRACT

The present invention provides a method and system which is useful on instruments requiring experimentally determined calibration curves by which calibration data can be transferred to a plurality of field gauges, thereby avoiding the necessity of individually calibrating each gauge each time calibration is necessary. The field gauges are initially cross related to a master gauge. At a later time when a new calibration is necessary, the master gauge is calibrated using carefully prepared samples of a test material. Using the experimentally derived calibration curves with the cross relation data provides calibration data for the field gauges.

FIELD OF THE INVENTION

This invention relates to measurement instruments requiringexperimentally determined calibration curves, where minor variations ininstrument characteristics necessitate individual calibration, and moreparticularly relates to a method and system which facilitates thecalibration of such instruments.

BACKGROUND OF THE INVENTION

Many types of measurement instruments rely upon experimentallydetermined calibration curves to convert the raw data which is read bythe instrument into an accurate measurement reading. Typically, thecalibration curve is derived by taking measurement readings with theinstrument on several samples whose composition has been determinedanalytically, and then constructing a calibration curve which relatesthe experimentally determined measurement readings to the analyticallydetermined composition values. Because of minor variations from onemeasurement instrument to another, a calibration curve is unique for aparticular instrument, and it is therefore necessary for eachmeasurement instrument to be calibrated individually. The presentinvention provides a method and system which greatly facilitates thecalibration procedure. This invention is described herein in terms ofthe calibration of a neutron gauge designed for measuring the asphaltcontent of bituminous paving mixes. This invention can, however, beembodied in many different forms and can be used with other types anddesigns of instruments which employ experimentally derived calibrationcurves.

Lowery, et al. U.S. Pat. No. 3,492,475 discloses a portable nucleargauge which utilizes a fast neutron source and a thermal neutrondetector for determining the composition of a bulk material, such as abituminous paving mix, placed in a sample pan. This type of gauge reliesupon the neutron moderating characteristics of hydrogen atoms present inthe composition for determining, for example, the amount of asphalt in apaving mix or the amount of moisture in a building material. For thesedeterminations it is known that the amount of asphalt or the amount ofmoisture can be related to the hydrogen content of the material, and thehydrogen content of the material can be determined by subjecting thesample to radiation from a fast neutron source and detecting neutronswhich have been slowed or thermalized as a result of interaction withthe hydrogen nuclei present in the sample. The number of thermalizedneutrons detected (counted) over a period of time is utilized indetermining the hydrogen content of the sample.

In operating the gauge, it is first necessary to establish a standardcount for calibration purposes. This is accomplished using a standardsample having a known hydrogen content, for example, a block ofpolyethylene. Then calibration curves are produced for the particularmaterial being tested, by using carefully prepared samples having aknown content of the hydrogen-containing material of interest (e.g.asphalt or moisture). After the calibration curves have been produced,unknown test samples can be placed in the gauge and counts are taken. Byreference to the calibration curve, the corresponding content of thehydrogen-containing material for that count can be read.

A more recent model of this gauge has been produced by applicant'sassignee embodying the principles of the Lowery patent and sold as the"Model 3241 Asphalt Content Gauge" by Troxler Electronic Laboratories,Inc. This gauge includes a microprocessor to facilitate calibration andcomputation of the sample asphalt content. Calibration can be made bytaking gauge counts on two or more samples of known asphalt content. Themicroprocessor then constructs a calibration equation from these datapoints, and the gauge provides a direct readout of the percent asphalt,thus eliminating the necessity of calculations and reference to externalcalibration tables.

In order to obtain the most accurate measurements, the gauge must becalibrated each time the composition of the material is changed. This isbecause the number of counts recorded is only representative of thehydrogen atoms present in the sample. There is an assumption made whenusing a thermal neutron gauge that the differences in hydrogen countfrom sample to sample are because of changes in the amount of thesubstance of interest, such as moisture or asphalt content, and that allother factors are maintained substantially constant. The calibration isdone when it is clear that the "other factors" are not going to beconstant. Such changes may occur, for example, when using a newaggregate in the paving mix or a new source or grade of asphalt. A newaggregate may have a different average moisture content or a differentintrinsic hydrogen content. In the case of asphalt, different sources ofasphalt may have a different concentration of hydrogen. At a time whenthere is such a change, the gauge must be calibrated using carefullyprepared samples of known concentrations of the hydrogen-containingmaterial of interest.

As discussed above, the calibration procedure involves taking hydrogencounts with the gauge using several samples of known composition, andestablishing a correlation, (e.g. an equation or a calibration curve)which can be used to obtain a percent asphalt reading from the hydrogencounts obtained from a test sample of unknown composition. Thecalibration procedure itself is not unduly complex, and is practicalwith a single gauge or where a relatively few gauges are involved.However, where a number of field gauges are used, as is frequently thecase in many operations, the necessity of manually calibrating all thegauges becomes quite burdensome and time consuming. The gauges generallyneed to be taken out of the field and sent to a lab where samples of thenew aggregate can be carefully mixed and tested to get a propercalibration. This involves the inconvenience of the loss of use of thegauges during the time they are being calibrated, and also theinconvenience of having to transport the gauges back and forth from thelab.

With the foregoing in mind, it is an object of the present invention toovercome the problems and disadvantages of the prior practices discussedabove and to provide an improved system for calibrating gauges in asimpler and more time efficient manner without having to transport thegauges back to the lab.

SUMMARY OF THE INVENTION

The invention achieves the foregoing and other objects by providing anefficient system by which calibration data can be transferred to aplurality of field gauges, thereby avoiding the necessity ofindividually calibrating each gauge. The calibration data required bythe gauges is obtained by a master gauge typically kept at the lab. Thiscalibration data is easily transferred to the respective field gauges sothat the field gauges are permitted to stay in the field.

The process essentially comprises providing a master instrument (e.g. aneutron gauge), and at least one field instrument (e.g. a neutrongauge). Since each instrument has different measurement characteristics,a cross relationship is established between the readings obtained fromthe master instrument when measuring a particular material and thosedetected by the field instrument when measuring the same material.

When a calibration is necessary, due to the use of a new material sourcefor example, the conventional manual calibration procedure is carriedout in the lab on the master gauge and master calibration constants areestablished for the particular material. Adjusted calibration constants,specific for a particular field gauge, are created by adjusting themaster calibration constants based upon the previously established crossrelationship between the master gauge and the particular field gauge.The adjusted calibration constants are used in the field gauge to obtainmeasurements on the new material.

In accordance with one embodiment of the invention, the field gauges arespecially equipped with means for storing the previously derived crossrelationship between the master gauge and the field gauge, and means isprovided in the field gauge for directly receiving master calibrationconstants obtained from the master gauge. The gauge is also equippedwith means for applying the stored cross relationship to the newlyobtained master calibration constants to create adjusted calibrationconstants specific for the particular field gauge. Thus whenever acalibration is necessary, such as when a new variation of asphalt isused, the master calibration constants are derived in the laboratory bythe master gauge, and these newly derived master calibration constantsare then distributed to the field gauges in use. The master calibrationconstants are loaded into each field gauge, and in each field gauge themaster calibration data is adjusted based upon the unique crossrelationship data which is stored in the field gauge. This is mucheasier and quicker than requiring individual calibration of each fieldgauge.

However, the calibration data transfer procedure of this invention canalso be utilized in instruments which are not specially equipped forcalibration data transfer, such as for example the asphalt contentgauges noted earlier, which have been produced by applicant's assigneefor many years. For use in these gauges, the master calibrationconstants are obtained in the laboratory on a master gauge and crossrelationships between each field gauge and the master gauge areestablished in the manner noted above. Then the master calibrationconstants are adjusted for each field gauge using the previously derivedcross relationships for each field gauge. This can be accomplishedmanually or preferably through the use of a computer. Then the adjustedcalibration constants for each gauge are distributed to the respectivefield gauges and loaded into the appropriate field gauge for use inperforming subsequent measurements. This permits the field units to stayin the field and avoids the time consuming process of individuallycalibrating each field gauge.

BRIEF DESCRIPTION OF THE DRAWINGS

Some of the features and advantages of the invention having been stated,others will become apparent as the description proceeds, and taken inconnection with the accompanying drawings, in which

FIG. 1 is a perspective view of a neutron gauge;

FIG. 2 is a perspective view of several sample pans filled with samplesto be tested in the neutron gauge;

FIG. 3 is a front cross section view of the neutron gauge of FIG. 1illustrating its basic components;

FIG. 4 is a graph illustrating the general relationship of thermalneutron counts to the asphalt content of a sample of asphalt-aggregatepaving mix and graphically representing the calibration of a thermalneutron gauge;

FIG. 5 is a flow chart illustrating the basic procedures followed by thepresent invention;

FIG. 6 is a flow diagram illustrating the detailed procedures pursuantto one embodiment of the present invention where specially equippedfield gauges are employed; and

FIG. 7 is a flow diagram similar to FIG. 6 illustrating the detailedprocedure of an alternate embodiment of the invention where standardfield gauges are employed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully with reference tothe drawings, in connection with a particular type of neutron gaugedesigned for measuring the asphalt content of bituminous paving mixes.This invention can, however, be embodied in many different forms and canbe used with other types and designs of instruments which useexperimentally determined calibration curves. It should be understoodtherefore that the specific embodiments described herein areillustrative of how the present invention may be practiced, and that theinvention is not limited to these specific embodiments.

A neutron gauge is generally indicated by the number 10 in FIG. 1 andcomprises a generally rectangular housing 11 having a door 12 whichprovides access to a measurement chamber in which sample pans are placedfor measurement. A control unit 14 is provided, including a keypad 15for entry of data and for controlling the functions of the gauge, and adisplay 16, which may be of any suitable construction, such as a liquidcrystal display. Referring to FIG. 2, there is shown several sample pans17 containing samples of asphalt-aggregate paving mix. The sample pansare sized to fit into the measurement chamber of the neutron gauge.Referring to FIG. 3, a sample pan 17 is received within the interior ofthe gauge. Located in the upper interior portion of the gauge is asource 20 of fast neutrons. The source 20 may for example suitablycomprise a Am-241:Be source. In the lower interior portion of the gaugebeneath the sample pan are a series of detector tubes 21 for detectingneutrons which have been slowed or thermalized by interaction withhydrogen atoms present in the sample. The illustrated detectors 21 areHe³ detector tubes but .[.an.]. .Iadd.any .Iaddend.suitable thermalneutron detector will suffice. The gauge also includes a data processormodule 23 for controlling the gauge and counting of thermalizedneutrons.

To operate the gauge, the sample pan is filled with a sample of materialand inserted into the interior of the gauge. The door is shut and fastneutrons from the source 20 are emitted down through the sample in thesample pan 13. Hydrogen present in the sample interacts with the fastneutrons, producing moderated or slowed neutrons, and thermalizedneutrons below a specified energy level are detected by detectors 21.The thermalized neutrons are counted for a predetermined period of timeand a count is recorded in the data processor module 23. The dataprocessor module 23 then correlates the number of counts to a moisturecontent or an asphalt content calibration to indicate the result.

The correlation between counts and asphalt content is unique for eachgauge. This is because each fast neutron source 20 emits neutrons at itsown particular rate and the detectors also have variations in efficiencyand design from unit to unit. Therefore, each gauge must be calibratedin order that the data processor module 23 can convert the number ofcounts into a value for the asphalt content of the sample. To calibratethe gauge in accordance with conventional methods known in the art,several samples are carefully prepared with known asphalt contents andare used in the gauge to generate counts. The correlation can be done inseveral different ways. For example, as shown in FIG. 4 the relationshipbetween observed counts and known asphalt content can be graphed. Then,a linear or other form of equation can be formulated to fit the data.Other ways include the creation of a "look up" table where the variousasphalt contents are cross referenced with a number of counts.

Calibration is best and most easily accomplished in the lab. This way,the known sample mixtures can be carefully prepared and the most precisecalibration can be obtained. However, if the user has a number of thesegauges in use in the field, which is often the case, returning thegauges to a lab each time calibration becomes necessary is mostinconvenient and would seriously interfere with the user's operations.

The present invention eliminates the necessity of returning field gaugesto the lab for calibration by providing a system by which calibrationdata can be transferred from a lab-based master gauge to one or morefield gauges. Illustrated in FIG. 5 is the general process of thesystem. The first step 31 is to establish a cross relationship whichestablishes the variance between the thermal neutron counts detected bythe master gauge and the counts detected by the field gauge whenmeasuring the same sample. This is accomplished by taking counts onvarious samples with both the master gauge and the field gauge. Thecomposition of the samples is not critical, although it is desirablethat the samples have a hydrogen content generally similar to that ofthe material which are to be measured during use of the gauge. Mostdesirably, several samples are used having a hydrogen content whichspans the range of measurement of the gauge. For example, standardblocks of solid polyethylene or polyethylene/metal laminates such asthat shown in commonly-owned U.S. Pat. No. 4,152,600 may be employed.The second step 32 involves performing a conventional calibrationprocedure with the use of the lab-based master gauge to obtain mastercalibration constants. This calibration procedure would be carried outwhenever calibration is required, such as due to the use of a new typeor variation of paving mix. In order for the master calibrationconstants to be usable in the field gauge, they must be adjusted orconverted to take into account the differences in measurement betweenthe field gauge and the master gauge. As indicated at 33 in FIG. 5,adjusted calibration constants are created by applying the previouslyderived cross relationship between the master gauge and field gauge tothe master calibration constants to thereby obtain adjusted calibrationconstants specific for the particular field gauge. The final step 34 ofthe process is to use the adjusted calibration constants in the fieldgauge on the material to obtain measurements of the amount of theconstituent of interest.

In accordance with one embodiment of the present invention, thecalibration data transfer procedure is used on gauges which arespecially equipped to store the previously defined master gauge/fieldgauge cross relationship and to receive unmodified calibration constantsfrom the master gauge and to internally adjust the constants based uponthe stored master gauge/field gauge cross relationship to produceadjusted calibration constants which are specific for the particularfield gauge and which can be used thereafter for determining percentasphalt based upon a thermal neutron count.

For this purpose, the data processor module 23 includes a storedcalibration transfer procedure or subroutine which can be calledwhenever the calibration transfer procedure is to be run. This procedurepermits manual entry of the master gauge/field gauge cross relationshipby the operator and stores this data in memory for subsequent use. Italso permits entry by the operator of the new master calibrationconstants, either manually or via a suitable transfer media such asmagnetic disk or EPROM. Additional data, such as background readings,explained more fully below, can also be entered at this time. Afterentry of all needed data, the calibration transfer subroutine carriesout a mathematical computation to adjust the master calibrationconstants based upon the stored master gauge/field gauge crossrelationship to create adjusted calibration constants which arethereafter stored and used by the field gauge in converting thermalneutron counts into values for percent asphalt. The method and apparatusin accordance with this embodiment of the present invention isadvantageous in that the calibration procedure is quite simple and isessentially automated. Since the master gauge/field gauge crossrelationship is stored in the field gauge, accuracy is assured inconverting or adjusting the master calibration constants to establishadjusted constants for the specific field gauge.

When the calibration data transfer procedure of the present invention isused with conventional thermal neutron gauges which are not speciallyequipped for receiving and internally storing the master gauge/fieldgauge cross relationship, the adjustment of the master calibrationconstants is performed before the calibration data is physicallytransferred to the field gauge. This may be suitably accomplished at thelaboratory either manually or by a computer program which executes aprocedure or subroutine similar to that described above. After adjustingthe master calibration constants using appropriate master gauge/fieldgauge cross relationship, the adjusted calibration constants are thenphysically transferred to the appropriate field gauge. Depending uponthe specific gauge and how it is designed to receive calibration data,the entry of the adjusted calibration data into the field gauge may beby manual entry or by other means, such as electronically.

The procedure in accordance with the first embodiment of the inventionis illustrated in more detail in FIG. 6. The broad steps or operationsdescribed above with reference to FIG. 5 are shown in the broken lineboxes and bear the same reference numbers. The more detailed steps oroperations are shown in the solid line boxes. Thus, one step inestablishing the master gauge/field gauge cross relationship includestaking a background reading on each of the master gauge and fieldgauges, as indicated at 41. The background readings are to eliminate thepossible error for the day to day differences in the field and labconditions and also the changes that occur over time in the source 20.The background reading is made by taking a count without any sample inthe gauge. The master gauge original background reading is specified asMOBG and the field gauge original background is specified as FOBG. Asearlier discussed, several samples are measured by the master and fieldgauges as indicated at 42 and a cross relationship is established asindicated at 43. Preferably, the cross relationship is established byselecting a minimum of five samples covering the range of percentasphalt used. The readings from the five samples are recorded as R_(M1),R_(M2), R_(M3), R_(M4), and R_(M5) for the master gauge and R_(F1),R_(F2), R_(F3), .[.P_(F4) .]. .Iadd.R_(F4) .Iaddend., and R_(F5) for thefield gauge. A cross relationship between the two gauges can now beestablished by fitting the counts from one gauge against the other.Please note that only the linear form of this process is consideredhere, but this procedure can be performed with other equations. Thus,

    R.sub.Mj =E.sub.1 +E.sub.2 R.sub.Fj

where j=1, 2 . . . 5. The cross relationship, which includes E₁, E₂,MOBG and FOBG, is stored in the field neutron gauge or more particularlythe central processing module 23, as indicated at 44.

At subsequent times, when it is necessary to calibrate a field gauge,which is most often done when a different type or variety of material isused, calibration is performed using the master gauge. The master gaugeis used to generate a background count on the empty gauge chamber asindicated at 45. The background count is specified as MBG. The mastergauge is then used to test carefully prepared samples of a particularvariety of the asphalt-aggregate paving mix, as indicated at 46, and thesamples are used to generate master calibration constants as indicatedat 47. A minimum of two samples are employed covering the range ofasphalt used. This will give readings R₁ and R₂. The counts R₁ and R₂are now used with the known asphalt content samples to establish themaster calibration constants A₁ and A₂, using the relationship

    %AC=A.sub.1 +A.sub.2 R.sub.M                               (1)

where R_(M) is master gauge count and %AC is asphalt content.

The master calibration constants, which include A₁, A₂ and MBG, are thentransferred and input into the field neutron gauge, or more particularlythe central processing module 23 as indicated at 48. Then as indicatedat 49, the field gauge creates adjusted calibration constants AA₁ andAA₂ by adjusting the master calibration constants A₁ and A₂ based on thecross relationship stored in the field gauge.

The following discussion explains how the adjusted calibration constantsare derived. Using the equation

    R.sub.M =E.sub.1 +E.sub.2 R.sub.F                          (2)

to account for any changes in the gauge counts since the time of crosscalibration the stored background counts have to be used in the aboveequation, so

    R.sub.M +(MOBG-MBG)=E.sub.1 +E.sub.2 [R.sub.F +-(FOBG-DBG)](3)

wherein DBG is the field gauge daily background count. R_(M) iscalculated Master Gauge count, and R_(F) is the measured Field GaugeCount. Rewriting equation (3)

    R.sub.M =E.sub.1 +E.sub.2 [R.sub.F +(FOBG-DBG)]+MBG-MOBG   (4)

For simplicity let

    F.sub.1 =MBG-MOBG

and

    R.sub.F *=R.sub.F +(FOBG-DBG)

now

    R.sub.M =E.sub.1 +E.sub.2 R.sub.F *+F.sub.1

Substitute R_(M) into equation 1 to get

    %AC=A.sub.1 +A.sub.2 (E.sub.1 +E.sub.2 R.sub.F *+F.sub.1)

    %AC=A.sub.1 +A.sub.2 E.sub.1 +A.sub.2 E.sub.2 R.sub.F *+A.sub.2 F.sub.1

or

    %AC=(A.sub.1 +A.sub.2 E.sub.1 +A.sub.2 F.sub.1)+(A.sub.2 E.sub.2)R.sub.F *

let

    AA1=A.sub.1 +A.sub.2 E.sub.1 +A.sub.2 F.sub.1

and

    AA2=A.sub.2 E.sub.2

Finally, the constants stored in the Field Gauge are AA1 and AA2.

In use, daily background measurements specified as DBG are taken fromthe field gauge as indicated at 50 and the field gauge is used to obtainmeasurements of the asphalt content of an asphalt-aggregate paving mixas indicated at 51 such that

    %AC=AA.sub.1 +AA.sub.2 (R.sub.F +FOBG-DBG)

The process of transferring the calibration to the standard gauges issubstantially similar to the process described above, and is illustratedin FIG. 7. To avoid repetition, the procedures or steps shown in FIG. 7which correspond to those previously described in FIG. 6 are identifiedwith corresponding reference characters, with prime notation added.Basically, the fundamental difference in this procedure is that theadjusted calibration constants AA₁, AA₂ for the field gauge are producedoutside of the field gauge (e.g. at the laboratory). Then the adjustedcalibration constants AA₁, AA₂ (rather than the master calibrationconstants) are transferred to the field gauge as indicated at 48' inFIG. 7.

That which I claim is:
 1. A test method for use with measurementinstruments of the type which obtain measurement data from a sample andwhich utilize experimentally determined calibration curves to convertthe measurement data into measurement readings, said test method beingcharacterized by facilitating the calibration and use of a number offield instruments, and comprising the steps ofproviding a mastermeasurement instrument; providing at least one field measurementinstrument; establishing a cross relationship between the measurementdata detected by the master instrument and the measurement data detectedby the field instrument; .Iadd.obtaining a background measurement by themaster instrument; .Iaddend. establishing master calibration data for aparticular material by testing samples using the master instrument;creating adjusted calibration data, specific for a particular fieldinstrument, by adjusting the master calibration data based upon thepreviously established cross relationship between the master instrumentand that particular field instrument .Iadd.and the previously obtainedmaster instrument background measurement.Iaddend.; .Iadd.obtaining abackground measurement by the field instrument; .Iaddend.and using theadjusted calibration data in the field instrument .Iadd.and thebackground measurement obtained by the field instrument .Iaddend.toconvert measurement data obtained by the field instrument intomeasurement readings.
 2. The method according to claim 1, wherein thestep of creating adjusted calibration data comprises storing the crossrelationship between the master instrument and the field instrument inthe field instrument, transferring the calibration data of the masterinstrument to the field instrument, and applying the stored crossrelationship to the master calibration data to create adjustedcalibration data in the field instrument for use in converting themeasurement data obtained by the field instrument into measurementreadings.
 3. The method according to claim 1, wherein the step ofcreating adjusted calibration data comprises applying the previouslyestablished cross relationship between the master instrument and thefield instrument to the master calibration data to create the adjustedcalibration data, transferring the thus created adjusted calibrationdata to the field instrument, and storing the adjusted calibration datain the field instrument for use in converting the measurement dataobtained by the field instrument into measurement readings.
 4. Themethod according to claim 1, wherein the step of establishing a crossrelationship between the master instrument and the field instrumentcomprises obtaining measurement data for a plurality of samples usingthe master instrument, obtaining measurement data for the same pluralityof samples using the field instrument, and defining a relationshipbetween the measurement data obtained by the field instrument and themeasurement data obtained by the master instrument; and wherein saidstep of creating adjusted calibration data comprises applying the thusdefined relationship to the master calibration data to thereby derivethe adjusted calibration data for the field instrument.
 5. A test methodfor use with nuclear gauges of the type which measure the neutronmoderating characteristics of a sample by detecting thermal neutroncounts, and through the use of calibration constants for a particulartype of material, provide a measurement of the amount of a hydrogenoussubstance in a sample of the material, said test method beingcharacterized by facilitating the calibration and use of a number offield gauges, and comprising the steps ofproviding a master neutrongauge; providing at least one field neutron gauge; establishing a crossrelationship between the thermal neutron counts detected by the mastergauge and those detected by the field gauge; .Iadd.obtaining abackground measurement by the master gauge; .Iaddend. establishingmaster calibration constants for a particular material using the mastergauge; creating adjusted calibration constants, specific for aparticular field gauge by adjusting the master calibration constantsbased upon the previously established cross relationship between themaster gauge and that particular field gauge .Iadd.and the previouslyobtained master gauge background measurement.Iaddend.; .Iadd.obtaining abackground measurement by the field gauge; .Iaddend.and using theadjusted calibration constants in the field gauge .Iadd.and thebackground measurement obtained by the field gauge .Iaddend.to obtainmeasurements of the amount of the hydrogenous substance in a sample ofthe material.
 6. The method according to claim 5, wherein the stepcreating adjusted calibration constants comprises storing the crossrelationship between the master gauge and the field gauge in the fieldgauge, transferring the calibration constants of the master gauge to thefield gauge, and applying the stored cross relationship to the mastercalibration constants to create adjusted calibration constants in thefield gauge for use in obtaining measurements of the amount of saidhydrogenous substance in a sample of material.
 7. The method accordingto claim 5, wherein the step of creating adjusted calibration constantscomprises applying the previously established cross relationship betweenthe master gauge and the field gauge to the master calibration constantsto create the adjusted calibration constants, transferring the thuscreated adjusted calibration constants to the field gauge, and storingthe adjusted calibration constants in the field gauge for use inobtaining measurements of the amount of said hydrogenous substance in asample of material.
 8. The method according to claim 5, wherein the stepof establishing a cross relationship between the master gauge and thefield gauge comprises obtaining measurements of a plurality of samplesby the master gauge, obtaining measurements of the same plurality ofsamples by the field gauge, and defining a relationship between themeasurements obtained by the field gauge and those obtained by themaster gauge; and wherein said step of creating adjusted calibrationconstants comprises applying the thus defined relationship to the mastercalibration constants generated on the master gauge to thereby derivethe adjusted calibration constants for the field gauge.
 9. The methodaccording to claim 5, wherein said step of establishing a crossrelationship includes the step of establishing an initial backgroundmeasurement by each of the master gauge and field gauge .[.and furtherwherein said step of establishing calibration constants includesobtaining a subsequent background measurement by the master gauge andsaid step of creating adjusted calibration constants also includesobtaining a subsequent background measurement by the field gauge.].. 10.A test method for measuring the asphalt content of an asphalt-aggregatepaving mix with the use of nuclear gauges of the type which measure theneutron moderating characteristics of a sample of the asphalt-aggregatemix and obtain thermal neutron counts which represent, through the useof calibration constants, a measurement of the asphalt content of asample of the asphalt-aggregate mix, said method characterized byfacilitating the calibration and use of a number of field gauges andcomprising the steps ofproviding a master neutron gauge; providing atleast one field neutron gauge; .Iadd.establishing initial backgroundmeasurements by the master gauge and by the at least one field gauge;.Iaddend. establishing a cross relationship between the thermal neutroncounts detected by the master gauge and those detected by the fieldgauge when measuring the asphalt content of a sample; .Iadd.obtaining asubsequent background measurement by the master gauge for use incomparison against the initial master gauge background measurement toadjust for changes in counts since establishment of said crossrelationship; .Iaddend. establishing master calibration constants for aparticular variety of asphalt-aggregate paving mix using the mastergauge; generating adjusted calibration constants for the particularvariety of asphalt-aggregate paving mix which are specific for aparticular field gauge by adjusting the master calibration constantsbased upon the previously established cross relationship between themaster gauge and that particular field gauge .Iadd.and the previouslyestablished initial master gauge background measurement.Iaddend.;.Iadd.obtaining a subsequent background measurement by the field gauge;.Iaddend.and using the adjusted calibration constants in the field gauge.Iadd.and the initial and subsequent background measurement obtained bythe field gauge .Iaddend.to obtain measurements of the asphalt contentof the particular variety of asphalt-aggregate paving mix.
 11. Themethod according to claim 10, wherein the step of generating adjustedcalibration constants comprises storing the cross relationship betweenthe master gauge and the field gauge in the field gauge, transferringthe calibration constants of the master gauge to the field gauge,applying the stored cross relationship to the master calibrationconstants to create adjusted calibration constants in the field gauge,and storing the thus created adjusted calibration constants in the fieldgauge for use in obtaining measurements of the asphalt content of asample of the asphalt-aggregate paving mix.
 12. The method according toclaim 10, wherein the step of generating adjusted calibration constantscomprises applying the previously established cross relationship betweenthe master gauge and the field gauge to the master calibration constantsto generate the adjusted calibration constants, transferring the thusgenerated adjusted calibration constants to the field gauge, and storingthe adjusted calibration constants in the field gauge for use inobtaining measurements of the asphalt content in an asphalt-aggregatepaving mix.
 13. The method according to claim 10, wherein the step ofestablishing master calibration constants for a particular variety ofasphalt-aggregate paving mix comprises using the master gauge to obtainthermal neutron counts for a plurality of samples of the paving mixhaving known asphalt contents.
 14. A test method for measuring theasphalt content of an asphalt-aggregate paving mix with the use ofnuclear gauges of the type which measure the neutron moderatingcharacteristics of a sample of the asphalt-aggregate paving mix bydetecting thermal neutron counts, wherein the gauges are calibratedthrough the use of calibration constants for a particular variety ofasphalt-aggregate paving mix and the gauges provide a measurementindicative of the asphalt content in a sample of the particular varietyof paving mix, said method being characterized by facilitating thecalibration and simultaneous use of a number of field gauges, andcomprising the steps ofproviding a .[.lab based.]. master neutron gauge;providing at least one field neutron gauge; establishing a crossrelationship between the thermal neutron counts detected by a mastergauge and those detected by the field .Iadd.gauge .Iaddend.by obtaininga background thermal neutron count by the master gauge and thermalneutron counts of a plurality of samples of different compositions bythe master gauge, also obtaining a background thermal neutron count bythe field gauge and thermal neutron counts of the same plurality ofsamples, and defining the cross relationship between the thermal neutroncounts obtained by the master gauge and those obtained by the fieldgauge; storing the thus established cross relationship in the fieldgauge; establishing master calibration constants for a particularvariety of the asphalt-aggregate mix by using the master gauge to obtainthermal neutron counts for samples of known asphalt content;establishing a master background measurement on the master gauge;transferring the master calibration constants and the master backgroundmeasurement to the field gauge; generating adjusted calibrationconstants for the particular variety of asphalt-aggregate paving mixwhich are specific for the particular field gauge by adjusting themaster calibration constants based upon the cross relationship which isstored in the field gauge, and using the adjusted calibration constantsin the field gauge to obtain measurements of the asphalt content of theparticular asphalt-aggregate paving mix.
 15. A test method for measuringthe asphalt content of an asphalt-aggregate paving mix with the use ofnuclear gauges of the type which measure the neutron moderatingcharacteristics of a sample of the asphalt-aggregate paving mix bydetecting thermal neutron counts, wherein the gauges are calibratedthrough the use of calibration constants for a particular variety ofasphalt-aggregate paving mix and the gauges provide a measurementindicative of the asphalt content in a sample of the particular varietyof paving mix, said method being characterized by facilitating thecalibration and simultaneous use of a number of field gauges, andcomprising the steps ofproviding a .[.lab based.]. master neutron gauge;providing at least one field neutron gauge; establishing a crossrelationship between the thermal neutron counts detected by a mastergauge and those detected by the field .Iadd.gauge .Iaddend.by obtaininga background thermal neutron count using the master gauge and thermalneutron counts of a plurality of samples of different compositions bythe master gauge, also obtaining a background thermal neutron countusing the field gauge and thermal neutron counts of the same pluralityof samples using the field gauge, and defining the cross relationshipbetween the measurements obtained by the master gauge and those obtainedby the field gauge; storing the thus established cross relationship;establishing master calibration constants for a particular variety ofthe asphalt-aggregate mix by using the master gauge to obtain thermalneutron counts for samples of known asphalt content; establishing amaster background measurement on the master gauge; generating adjustedcalibration constants for the particular variety of asphalt-aggregatepaving mix which are specific for the particular field gauge byadjusting the master calibration constants based upon the crossrelationship; transferring the adjusted calibration constants to thefield gauge; and using the adjusted calibration constants in the fieldgauge to obtain measurements of the asphalt content of the particularasphalt-aggregate paving mix.
 16. A test system for measurementinstruments of the type which obtain measurement data from a sample andwhich utilize experimentally determined calibration curves to convertthe measurement data into measurement readings, said test system beingcharacterized by facilitating the calibration and use of a number offield instruments, and comprisinga master measurement instrument; atleast one field measurement instrument; means for storing a derivedcross relationship between the measurement data detected by the masterinstrument and the measurement data detected by the field instrument;means for storing master calibration data derived from .[.tests.]..Iadd.measurements .Iaddend.with the master measurement instrument on aparticular material; means for applying the stored cross relationship tothe stored master calibration data to create adjusted calibration data;and means in the particular field instrument for using the adjustedcalibration data in the field instrument .Iadd.and a backgroundmeasurement made by the field instrument .Iaddend.to convert measurementdata obtained by the field instrument into measurement readings.
 17. Thesystem according to claim 16, wherein the means for using the adjustedcalibration data also includes means for storing the cross relationshipbetween the master instrument and the field instrument and for receivingthe recorded master calibration data and for receiving the adjustedcalibration data.
 18. The system according to claim 16, wherein themeans for storing and using the adjusted calibration constants alsoincludes means for receiving the adjusted calibration data.
 19. Thesystem according to claim 16, further includingmeans for recordingmeasurement data by the master instrument for a plurality of samples;means for recording measurement data by the field instrument for thesame plurality of samples; and means for deriving a cross relationshipbetween the measurement data obtained by the master instrument and themeasurement data obtained by the field instrument.
 20. A test system fornuclear gauges of the type which measure the neutron moderatingcharacteristics of a sample, and through the use of calibrationconstants determined for each particular material a gauge may provide ameasurement of the amount of a hydrogenous constituent in a sample ofthe material, wherein the calibration of a plurality of field gauges isfacilitated by the system comprisinga master neutron gauge; at least onefield neutron gauge; means for storing a derived cross relationshipbetween a field gauge and a master gauge defining the variance betweenthe thermal neutron counts detected by the master gauge those detectedby the field gauge .[.between the gauges.]. for a sample; means forstoring .[.derived.]. master calibration constants .Iadd.derived frommeasurements with the master gauge .Iaddend.for a particular material;means for applying the stored cross relationship to the stored mastercalibration constants to create adjusted calibration constants; andmeans in the particular field gauge for .[.storing and.]. using.Iadd.the thus created .Iaddend.adjusted calibration constants .Iadd.anda background measurement made by the field gauge .Iaddend.to measure theamount of a hydrogenous constituent in a sample of the particularmaterial.
 21. The system according to claim 20, wherein the means forstoring and using the adjusted calibration constants also includes meansfor storing the cross relationship between the measurements by themaster gauge and those by the field gauge and for receiving the recordedmaster calibration constants and for deriving the adjusted calibrationconstants.
 22. The system according to claim 20, wherein the means forstoring and using the adjusted calibration constants also includes meansfor receiving the adjusted calibration constants.
 23. The systemaccording to claim 20, further includingmeans for recording thermalneutron counts by the master gauge for a plurality of samples; means forrecording thermal neutron counts by the field gauge for the sameplurality of samples; means for deriving a cross relationship betweenthe thermal neutron counts detected by the master gauge and thosedetected by the field gauge.
 24. A test system for nuclear gauges of thetype which measure the neutron moderating characteristics of a sample ofan asphalt-aggregate paving mix, and through the use of calibrationconstants determined for each particular asphalt-aggregate paving mix agauge may provide a measurement of the asphalt content of a sample ofthe asphalt-aggregate paving mix, wherein the calibration of a pluralityof field gauges is facilitated by the system comprisinga master neutrongauge; at least one field neutron gauge; means for recording derivedmaster calibration constants for a particular asphalt-aggregate pavingmix; means in the particular field gauge for storing a crossrelationship between the thermal neutron counts detected by the mastergauge and those detected by the field gauge; means in the particularfield gauge for receiving the recorded master calibration constants andfor applying the stored cross relationship to the master calibrationconstants to create adjusted calibration constants; and means in theparticular field gauge for using the thus-derived adjusted calibrationconstants to measure the asphalt content in a sample of the particularasphalt-aggregate paving mix.
 25. A test system for nuclear gauges ofthe type which measure the neutron moderating characteristics of asample of an asphalt-aggregate paving mix, and through the use ofcalibration constants determined for each particular variety ofasphalt-aggregate paving mix a gauge may provide a measurement of theasphalt content of a sample of the asphalt-aggregate paving mix, whereinthe calibration of a plurality of field gauges is facilitated by thesystem comprisinga master neutron gauge; at least one field neutrongauge; means for storing a derived cross relationship between thethermal neutron counts detected by the master gauge and those.Iadd.detected .Iaddend.by .[.each.]. .Iadd.a .Iaddend.field gauge.Iadd.based upon measurements of samples by the master gauge and thefield gauge.Iaddend.; means for storing derived master calibrationconstants for a particular asphalt-aggregate paving mix .Iadd.based uponmeasurements by the master gauge of samples of the particularasphalt-aggregate paving mix.Iaddend.; means for applying the storedcross relationship to the stored master calibration constants to createadjusted calibration constants; and means in the particular field gaugefor receiving, storing and using adjusted calibration constants.Iadd.and a background measurement made by the field gauge .Iaddend.tomeasure the asphalt content in a sample of the particularasphalt-aggregate paving mix. .Iadd.
 26. A test method for use withnuclear gauges of the type which measure the neutron moderatingcharacteristics of a sample by detecting thermal neutron counts, andthrough the use of calibration constants for a particular type ofmaterial, provide a measurement of the amount of a hydrogenous substancein a sample of the material, said test method being characterized byfacilitating the calibration and use of a number of field gauges, andcomprising the steps ofproviding a master neutron gauge; providing atleast one field neutron gauge; establishing a cross relationship betweenthe thermal neutron counts detected by the master gauge and thosedetected by a particular field gauge; storing in the particular fieldgauge the thus established cross relationship between the master gaugeand the particular field gauge; establishing master calibrationconstants for a particular material using the master gauge; transferringthe thus established master calibration constants for the particularmaterial to the field gauge; applying the cross relationship stored inthe field gauge to the thus transferred master calibration constants tocreate adjusted calibration constants in the field gauge specific forthe particular field gauge; and using the adjusted calibration constantsin the field gauge to obtain measurements of the amount of thehydrogenous substance in a sample of the material. .Iaddend. .Iadd. 27.A test method for measuring the asphalt content of an asphalt-aggregatepaving mix with the use of nuclear gauges of the type which measure theneutron moderating characteristics of a sample of the asphalt-aggregatepaving mix and obtain thermal neutron counts which represent, throughthe use of calibration constants, a measurement of the asphalt contentof a sample of the asphalt-aggregate paving mix, said test method beingcharacterized by facilitating the calibration and use of a number offield gauges, and comprising the steps ofproviding a master neutrongauge; providing at least one field neutron gauge; establishing a crossrelationship between the thermal neutron counts detected by the mastergauge and those detected by a particular field gauge when measuring theasphalt content of a sample; storing in the particular field gauge, thethus established cross relationship between the master gauge and theparticular field gauge; establishing master calibration constants for aparticular asphalt-aggregate paving mix using the master gauge;transferring the thus established master calibration constants for theparticular paving mix to the field gauge; applying the crossrelationship stored in the field gauge to the thus transferred mastercalibration constants to create adjusted calibration constants in thefield gauge specific for the particular field gauge; storing theadjusted calibration constants in the field gauge; and using theadjusted calibration constants in the field gauge to obtain measurementsof the asphalt content in a sample of the particular asphalt-aggregatepaving mix. .Iaddend.